1. Field of the Invention
The present invention concerns the fabrication of an optical preform. A preform of this kind is used to manufacture an optical fiber.
To be more precise, the invention concerns a method and system for detecting a critical state of closing of an optical preform during a phase of closing the preform following a phase of depositing layers of material that reduce the radial size of an opening formed in the preform. The preform fabrication technique is typically of the intra-tube type, such as the Modified Chemical Vapor Deposition (MCVD) technique.
2. Description of the Prior Art
Referring to FIG. 1, which is highly schematic, the initial phase of depositing layers of material for the fabrication of an optical preform is carried out in the following manner. Starting with a silica tube 1, layers 2 of material that will form the cladding and the optical core of the optical fiber subsequently extracted from the preform are deposited successively onto the interior wall of the tube 1. The layers 2 of material deposited inside the tube 1 are the result of chemical reactions between gaseous reagents 3 injected into the tube 1. A heating element 4 outside the tube 1 increases the temperature inside the tube 1 to a level sufficient for the required reactions between the gaseous reagents to take place. The following two reactions between silicon tetrachloride and oxygen, on the one hand, and between germanium tetrachloride and oxygen, on the other hand, are typical examples:
SiCl4+O2xe2x86x92SiO2+2Cl2;
and
GeCl4+O2xe2x86x92GeO2+2Cl2
Layer by layer deposition in this manner offers the facility for very precise shaping of the refractive index profile of the optical fiber that will be obtained from the preform, by appropriate metering of the reagents 3 injected into the tube 1. The depositing of these layers of material therefore reduces the radial size of an opening formed in the preform as a consequence of the increased thickness of the layers 2.
When all of the material needed to form the preform has been deposited in layers onto the inner wall of the tube 1 the deposition phase is stopped and is followed by a phase of closing the preform. For this closing phase, the objective of which is to close the preform completely, the temperature of the preform is increased and the feed of gaseous reagents is stopped. It is necessary to inject a gas under pressure into the opening formed in the preform, however, typically an oxygen-based gas. This gas must be injected under pressure only until a critical time corresponding to a critical state of closing of the preform. This critical state must be detected with great accuracy to prevent all risks of retrograde diffusion due to interrupting injection of the gas before the critical time and all risk of bursting of the preform if the injection of gas is continued after the critical time.
In the prior art, the only way to detect this critical state of closing of the preform is by human intervention. It is based on visual monitoring of the changing diameter of the opening formed in the preform. The fact that human intervention alone is relied on limits the reliability of the assessment of the critical time.
The invention is therefore directed to remedying this drawback by providing a method and system for detecting a critical state of closing of an optical preform.
The invention consists in a method of detecting a critical state of closing of a preform during a phase of closing of said preform following a phase of deposition of layers of material that reduce the radial size of an opening formed in said preform, said method comprising the steps of:
injecting a gas into said opening,
measuring a pressure in a pipe element through which the injected gas flows, and
detecting said critical state of closing of said optical preform on the basis of a measured pressure.
A valve for evacuating said injected gas is opened in response to said detection of said critical state of closing of said optical preform in order to reduce the pressure in the opening.
In a first embodiment the detection is effective when a difference between said measured pressure and a measured initial pressure is greater than a predetermined threshold.
In a second embodiment the detection is effective when said measured pressure is greater than a predetermined threshold.
The invention also consists in a system for implementing the method comprising:
pipe means connected to one end of said preform,
means for injecting said gas into said opening via said pipe means, and
sensor means for measuring a pressure in said pipe means.
The system advantageously further comprises suction pump means connected to the other end of said preform.
Other features and advantages of the present invention will emerge more clearly from a reading of the following description with reference to the corresponding appended drawings.